The present invention relates to optical fiber transmission systems and more particularly to a device for introducing dispersion into such a system to limit its bandwidth or information carrying capacity.
It is sometimes desirable to limit the capacity of an optical transmission system to some predetermined maximum data rate. For example, a customer may purchase the sole use of a single-mode optical fiber transmission line and connect its own terminal equipment thereto. The price of using the transmission line may be based on the maximum data rate that the customer intends to transmit. The bandwidth of the single-mode fiber transmission line may be much greater than that currently needed by the customer.
The bandwidth of such a system can be limited by inserting a suitable device in series with the transmission medium. In addition to limiting the bit rate, the device should possess some or all of the following attributes. It should not degrade the performance of the system at bit rates up to the set limit of the agreed upon bit rate. The insertion loss should be as small as possible. The device should be relatively insensitive to system length and wavelength. In addition, it should be simple and inexpensive to make and use.
Some proposed active devices include those which monitor the bit rate and render the system inoperable if the bit rate exceeds the maximum permissible rate. These active devices are complex and expensive.
Two relatively simple passive devices employ power splitters that branch off a portion of the bit power and reintroduce it with a predetermined delay. The splitter technique suffers from the disadvantage of wavelength dependence. Also, repeated coupling of progressively smaller amplitude pulses induced by the splitter at multiples of the primary delay time, may cause problems.
A passive data rate limiter is disclosed in U.S. patent application Ser. No. 894,576, filed Aug. 8, 1986 for "Data Rate Limiter for Optical Transmission System" (D. Charlton). The transmitted signal is connected to wavelength disperser means which spatially separates the different wavelength components of the transmitted signal. Each wavelength component is then propagated with a different delay by optical fiber delay means. The wavelength component which propagates with the least delay through the transmission optical fiber is subjected to the least induced delay in the optical fiber delay means and that which propagates with the greatest delay through the transmission optical fiber is subjected to the greatest induced delay in optical fiber delay means, the remaining components being subjected to intermediate delays. The delayed wavelength components are combined to form a series of broadened optica pulses. That device lacks simplicity and is wavelength dependent.